Network Termination type 1. The NT-1 is a device which converts the two-wire line (or "U" interface) coming from your telephone company into a 4-wire line (or "S/T" interface). The NT- 1 is physically connected between the ISDN board of your videoconferencing system and your ISDN phone line. The NT- 1 supports network maintenance functions such as loop testing. Check to see if your ISDN equipment requires an external NT-1 to operate.